SO42

Meaning: SO₄²⁻ is the sulfate ion. A central sulfur is bonded to four oxygens in a tetrahedral arrangement, and resonance makes the four S–O bonds equivalent. Sulfur is in the +6 oxidation state. Sulfate is the conjugate base of HSO₄⁻ (bisulfate), which comes from the strong acid H₂SO₄, so sulfate is a very weak base and is often a spectator ion. Most sulfate salts are soluble. BaSO₄, PbSO₄ and SrSO₄ are common exceptions, and CaSO₄ is only slightly soluble.

Equation: BaSO₄(s) ⇌ Ba²⁺(aq) + SO₄²⁻(aq), with Ksp = [Ba²⁺][SO₄²⁻]. Brackets mean molar concentration (mol/L), and Ksp is the solubility product, usually given without units.

On the MCAT: Sulfate appears in solubility-rule and precipitation questions, such as mixing Ba²⁺ or Pb²⁺ with a sulfate solution and predicting a solid forms. It also appears in common-ion problems, where adding Na₂SO₄ lowers the solubility of BaSO₄. Other uses are net ionic equations (sulfate as a spectator), naming and writing formulas, and finding the oxidation number of sulfur.

Common confusion: Do not mix up sulfate (SO₄²⁻, S = +6) with sulfite (SO₃²⁻, S = +4), or with sulfide (S²⁻). Also, HSO₄⁻ is still a fairly strong weak acid (Ka near 10⁻²), so NaHSO₄ solutions are acidic. Na₂SO₄ solutions are close to neutral because sulfate barely accepts protons from water.